Connected: An Internet Encyclopedia
RFC 1812 - 5.2.1.2 Unicast

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### 5.2.1.2 Unicast

Since the local delivery case is well covered by [INTRO:2], the following assumes that the IP datagram was queued for forwarding. If the destination is an IP unicast address:

       (5) The forwarder determines the next hop IP address for the packet,
            usually by looking up the packet's destination in the router's
            routing table.  This procedure is described in more detail in
            Section [5.2.4].  This procedure also decides which network
            interface should be used to send the packet.

       (6) The forwarder verifies that forwarding the packet is permitted.
            The source and destination addresses should be valid, as
            described in Section [5.3.7] and Section [5.3.4] If the router
            supports administrative constraints on forwarding, such as those
            described in Section [5.3.9], those constraints must be
            satisfied.

       (7) The forwarder decrements (by at least one) and checks the
            packet's TTL, as described in Section [5.3.1].

       (8) The forwarder performs any IP option processing that could not be
            completed in step 3.

       (9) The forwarder performs any necessary IP fragmentation, as
            described in Section [4.2.2.7].  Since this step occurs after
            outbound interface selection (step 5), all fragments of the same
            datagram will be transmitted out the same interface.

       (10) The forwarder determines the Link Layer address of the packet's
            next hop.  The mechanisms for doing this are Link Layer-
            dependent (see chapter 3).

       (11) The forwarder encapsulates the IP datagram (or each of the
            fragments thereof) in an appropriate Link Layer frame and queues
            it for output on the interface selected in step 5.

       (12) The forwarder sends an ICMP redirect if necessary, as described
            in Section [4.3.3.2].

Next: 5.2.1.3 Multicast

Connected: An Internet Encyclopedia
RFC 1812 - 5.2.1.2 Unicast